Helping the Earth Heal Itself

As of late, it hasn't grabbed the headlines like global warming, but the ozone hole still exists. In fact, it's even larger now than it was when its sudden appearance fifteen miles above the Antarctic shocked the scientific community in 1985.

Historically, the amount of ozone (O₃) in the stratosphere maintained itself via the ozone-oxygen cycle, a regenerative process in which ultraviolet (UV) light interacts with oxygen molecules (O₂), yielding O₃ and atomic oxygen, which can react further with each other to produce O₂. Since this process allows ozone to absorb most of the UV radiation from the sun, depletion of the O₃ molecule leads to more UV reaching the ground, causing such unhappy results as rising levels of melanoma and shrinking populations of plankton in the oceans.

The state of the ozone hole is improving, however. Recent reports by the U.N. indicate that it will hover around its current size for a few more years before beginning to shrink and, in about 50 years, stop forming altogether. The eventual disappearance of this manmade phenomenon will mark the end of a long road for scientists and will constitute a remarkable recovery for the environment.

"The ozone hole is a scientific success story," said college alumna Susan Solomon (Ph.D. '81, Chem). A senior scientist with the National Oceanic and Atmospheric Administration in Boulder, Colorado, Solomon has played a key role in the ozone's comeback. She both correctly predicted what was causing the ozone hole to form—chloro-fluorocarbon (CFC) reactions on polar stratospheric clouds—and led the expedition to Antarctica in 1986 that produced definitive evidence for the role of CFC's in ozone depletion.

Decreases in the ozone layer had been predicted since the 1970's. In 1970 Paul Crutzen realized that nitrogen oxides from fertilizers and supersonic aircraft could reduce the ozone layer. And in 1974 Frank Sherwood Rowland and Mario J. Molina realized that CFC's are very efficient catalysts for the breakup of ozone molecules. (All three scientists shared the Nobel Prize in Chemistry in 1995.)

However, scientists were stunned by the sheer magnitude of the hole that was eventually observed. Instead of losing just a few percentage points, the ozone over Antarctica dropped by over 50 percent, far more than was predicted by the computer models. Recalled Solomon, "The sheer magnitude of the ozone depletion—almost twice the size of the continental United States and orders of magnitude deeper than predicted based upon gas-phase chemistry—made many scientists at first think that the detection equipment must have been malfunctioning. Also, there was no immediate confirmation by satellite because the NASA satellite systems had a flag that discarded values thought to be wild points, and no one noticed that more and more of the data were being thrown out." Of course, once this was realized, the reanalyzed data indicated that the ozone depletion was indeed occurring on a large scale.

Susan Solomon's work has been key to our understanding of why the ozone hole forms in the Antarctic every year.

Solomon had studied how molecules form and move in the atmosphere since her graduate school days at Berkeley. However, her work had been confined to developing computer models of atmospheric chemistry and meteorology of molecular mixing. "When the ozone hole was discovered, my life really changed, and I stopped just doing work at the computer. [The discovery] was a bombshell, a tremendous mystery."

Pondering the problem—why such a big drop in ozone in the one place seemingly untouched by human pollution—she realized that the Antarctic environment itself must play a role. "Because of the coldness there, clouds can form within the Antarctic stratosphere; and when there are clouds, there are reactions that can take place at the surface of clouds that won't occur when the molecules are just bouncing around in the gas phase." She noted, "This was also viewed as a heretical idea at the time since it was all supposed to be gas phase chemistry. But the surface reaction that I put forward, hydrochloric acid with chlorine nitrate, does turn out to be a primary step in forming the ozone hole."

Solomon and her colleagues quickly realized that going to Antarctica was paramount to determining the root cause of the ozone depletion. So in August 1986, she led an expedition of 16 scientists to the Earth's southern-most continent. Taking careful chemical measurements of the ozone, she saw dropping levels as spring progressed. "In August we see a normal ozone layer, but then over a few weeks it just gets eaten alive [as the sunlight increases] and by mid-October has dwindled down to nothing."

Even though her work has had a great impact on scientific policy, she has not been outspoken about her results, preferring to let the science speak for itself. "We have put into the atmosphere six times more chlorine than would have normally been there, and these chlorinated molecules are very long lived. They don't get rained out or snowed out; they don't react with anything, and they have nowhere to go but up." When ozone-depleting chemicals reach the stratosphere, they are dissociated by ultraviolet light to release chlorine atoms, which then catalyze the destruction of many thousands of ozone molecules.

For her discoveries, she has been showered with honors, including election in 1992 as the youngest member at the time to the National Academy of Sciences, the 1999 National Medal of Science, the 2004 Blue Planet Prize and the naming of an iceberg in her honor.

She was inspired to go into science by another great environmental crusader, Jacques Yves Cousteau: "I loved that show and wanted to be a marine biologist." As an undergraduate student Solomon became interested in the chemistry of Jupiter and thought that studying the atmosphere of other planets would make for a good career. "It turns out that I never got off of Earth, and in fact I probably know less about Jupiter now than I did back when I was a student," she noted with a laugh.

After receiving her B.S. from the Illinois Institute of Technology, she came to Berkeley because "it was one of the best, if not the best, graduate school in chemistry." She spent the summer before enrolling at Berkeley doing research at the National Center for Atmospheric Research (NCAR) with Paul Crutzen, and her journey into atmospheric chemistry was complete. "I started at Berkeley in 1977 and had the good fortune of working with Harold Johnston, who had done some of the key early work on supersonic transport aircraft and their effect on the ozone layer," she said. Her thesis work, done jointly with Johnston and Crutzen, focused on the dynamics chemistry of molecules that are made high in the upper atmosphere by processes such as the aurora borealis.

Recently she has taken on a new challenge, serving as the Co-Chair of the U.N. Climate Change Panel. "The ability to be a bridge between the science community and the policy community is very exciting," she said.

She's also the author of The Coldest March, a book that explores from an atmospheric perspective the ill-fated attempt of Robert Falcon Scott's team to cross the Antarctic in 1912, drawing the conclusion that unusually cold weather played a part in the men's fate. The book was made into a television documentary in 2003. "Once you've been to the Antarctic, it's not just a place, it's an obsession," she explains.